Label the four layers of the digestive tract in the following figure.
<IMAGE>
a. ___
b. ___
c. ___
d. ___

Verified step by step guidance

1. Step 1: Understand the four main layers of the digestive tract wall, which from the innermost to the outermost are: mucosa, submucosa, muscularis externa, and serosa (or adventitia).
2. Step 2: Identify the innermost layer (a) that lines the lumen of the digestive tract; this is the mucosa, which includes the epithelium, lamina propria, and muscularis mucosae.
3. Step 3: Locate the layer just outside the mucosa (b), which is the submucosa; this layer contains connective tissue, blood vessels, nerves, and sometimes glands.
4. Step 4: Find the thick muscular layer (c) outside the submucosa; this is the muscularis externa, responsible for peristalsis and segmentation, typically composed of an inner circular and outer longitudinal muscle layer.
5. Step 5: Identify the outermost layer (d), which is the serosa if the digestive tract is intraperitoneal (a thin layer of connective tissue covered by mesothelium) or adventitia if retroperitoneal (connective tissue anchoring the tract to surrounding structures).

Key Concepts

Mucosa

The mucosa is the innermost layer of the digestive tract, responsible for secretion and absorption. It consists of an epithelial lining, a connective tissue layer called the lamina propria, and a thin layer of muscle called the muscularis mucosae.

Submucosa

The submucosa lies beneath the mucosa and is made of dense connective tissue containing blood vessels, nerves, and glands. It supports the mucosa and provides the digestive tract with elasticity and nourishment.

Muscularis externa

The muscularis externa is a thick layer of smooth muscle responsible for peristalsis and segmentation movements. It typically has an inner circular and an outer longitudinal muscle layer that coordinate to propel food through the digestive tract.